1. Field of the Invention
The present invention relates to a laser light source for emitting a plurality of laser light beams, a method for adjusting the focusing of the laser light source, and a scanning optical system.
2. Related Background Art
FIG. 1 illustrates the cross-sectional structure of a conventional laser scanning optical system taken along a meridional cross section in a main scan direction, and FIG. 2 the cross-sectional structure thereof taken along a sagittal cross section in an auxiliary scanning direction. In the laser scanning optical system, a semiconductor laser light emitting device 1, a collimator lens 2, a cylindrical lens 3, a polygonal mirror or a deflector 4, a f.theta. lens 5, and an illuminated surface 6 on which laser light is focused are arranged in this order to construct a fall-correction optical system. In the semiconductor laser light emitting device 1, a laser chip 8 is supported by a package 7.
Laser light emitted from the laser chip 8 is converted to approximately-parallel light by the collimator lens 2, and the laser light is then deflected by the deflector 4 and focused on the illuminated surface 6 by the f.theta. lens in the meridional cross-sectional plane. In the sagittal cross-sectional plane, the approximately-parallel light is once condensed onto a reflection surface of the polygonal mirror 4 by the cylindrical lens 3, and then focused on the illuminated surface 6 by the f.theta. lens.
FIG. 3 is a perspective view of the laser chip 8. In the semiconductor laser light emitting device 1, laser light emitted from a junction plane 1a of its active layer generally has a diverging characteristic, so that focus positions in directions perpendicular and horizontal to the junction surface 1a are apparently different from each other. That is, those focus positions in the perpendicular and horizontal directions are X and Y, respectively. A distance between the focus positions X and Y is called an astigmatism.
The astigmatism varies among individual semiconductor laser light emitting devices 1. Therefore, initially the collimator lens 2 is finely adjusted in a direction of its optical axis such that the best focusing can be obtained on the illuminated surface 6 in the meridional cross-sectional plane. Thereupon, the focal point of the collimator lens 2 coincides with the focus position in the direction horizontal to the junction plane 1a of the device 1, and the laser light after transmitted through the collimator lens 2 is converted to parallel light.
On the other hand, in the sagittal cross-sectional plane of FIG. 2, laser light emitted from the focus position in the direction perpendicular to the junction plane 1a is converted to a slightly-divergent light beam by the collimator lens 2 and converted to converging light by the cylindrical lens 3. At this time, the focus position on the illuminated surface 6 is adjusted by finely adjusting the cylindrical lens 3 in the direction of the optical axis.
Thus, manufacturing variations of focal lengths of the collimator lens 2, cylindrical lens 3 and f.theta. lens 5 together with the astigmatism of the semiconductor laser light emitting device 1 are absorbed, and the overall focus adjustment can hence be effected.
FIG. 4 illustrates the structure of another conventional example. The package 7 is omitted therein. A laser chip 8 in the semiconductor laser light emitting device 1 has focus positions X and Y in directions perpendicular and horizontal to the junction plane 1a, and a collimator lens 2 is positioned on the optical path.
Laser light emitted from the laser chip 8 is converted to parallel light by the collimator lens 2. Since the semiconductor laser light emitting device 1 has the astigmatism, the collimator lens 2 is adjusted along the optical axis such that a focal plane F of the collimator lens 2 is positioned at a middle point between the focus positions X and Y of the device 1.
Where the semiconductor laser light emitting device 1 is a single-beam laser light emitting device as in the above-discussed conventional example, the focus adjustment can be readily performed.
However, where the semiconductor laser light emitting device 1 is a multi-beam laser light emitting device, the focus adjustment cannot be executed to the full. This problem will be described with reference to FIG. 5.
FIG. 5 illustrates the structure of a multi-beam semiconductor laser light emitting device 11. In the multi-beam semiconductor laser light emitting device 1', a laser chip 8' is slantingly supported. Focus positions of the first laser light with respect to divergences in directions perpendicular and horizontal to a junction plane 1a' are points X1 and Y1, respectively, while those focus positions of the second laser light are points X2 and Y2, respectively.
Thus, there is a case where the astigmatism is different between the focus positions (X1 and Y1) of the first laser light and the focus positions (X2 and Y2) of the second laser light. Further, a positional variation of the laser chip 8' relative to the package 7' inevitably occurs within the manufacturing error. Hence, each light emission point is likely to deviate in the direction of the optical axis. Therefore, it is difficult to fully execute the focus adjustment solely by the positional adjustment of the collimator lens 2.